Method of producing oil from an oil shale formation

ABSTRACT

A method of producing hydrocarbons and optionally water-soluble minerals from a subterranean or underground oil shale formation containing zone(s) of water-soluble minerals by forming interconnecting cavities or a cavern within said water-soluble mineral-containing oil shale formation by fluid leaching the water-soluble minerals and simultaneously or sequentially flowing a hot fluid into the upper portion of the cavern in order to place the cavern roof including the cavern wall under stress so as to effect oil shale spalling and rubbling and improve recovery of hydrocarbons.

United States Patent 1 1111 3,739,851 Beard June 19, 1973 [5 METHOD OF PRODUCING OIL FROM AN 3,284,281 11/1966 Thomas 166/272 X 01 SHALE FORMATION 3,402,965 9/1968 Dahms et al..... 299/4 3,468,376 9/1969 Slusser et al..... 166/272 [75] Inventor: Thomas Nob e Be e Colo. 3,481,398 12/1969 Prats 166/271 x [73] Assignee: Shell Oil Company, Houston,Tex. OTHER PUBLICATIONS 2 Filed; No 2 19 1 How to Complete Thermal Wells Oil & Gas Journal,

Apr. 17, 1967, pp. 179 & 180 relied on. [21] Appl. No.: 201,941

Related [15. Application Data Primary ExaminerStephen J. Nlovosad [63] Continuation-impart of S61. N0. 860,349, Sept. 23, AtwneyrrGeorge Pmzker 1969, abandoned, which is a continuation of Ser. No. 770,964, Oct. 28, 1968, abandoned, and a [57] ABSTRACT contmuation-m-part of Ser. No. 75,009, Sept. 24, 9 0 A method of producing hydrocarbons and optionally water-soluble minerals from a subterranean or under- 52 US. Cl. 166/254, 166/271, 166/272, ground oil shale formation containing of water- 299/5 soluble minerals by forming interconnecting cavities or 51 1m, 211, 43 24, 2 1, 43 2 2 1, 3 2 a cavern within said water-soluble mineral-containing [58] Field of Search 166/272, 271, 303, oil shale formation y fluid leaching the water-Soluble 166/308, 248, 254, 258, 259, 269; 299/4, 5 minerals and simultaneously or sequentially flowing a hot fluid into the upper portion of the cavern in order 55 References Cited to place the cavern roof including the cavern wall UNITED STATES PATENTS under stress so as to effect oil shale spalling and rub- 2,969,226 1/1961 Huntington 166 272 x blmg and Improve recovery of hydrocarbons 3,050,290 8/1962 Caldwell et al. 18 Claims, 3 Drawing Figures 3,106,244 10/1963 Parker 166/248 PATENIEB Jmu 9:915

SEEEIIDFZ Q SEPARATOR HEAT EXCHANGER FL U105 IN METHOD OF PRODUCING OIL FROM AN OIL SHALE FORMATION CROSS-REFERENCE TO RELATED APPLICATIONS This application is a continuation-in-part of copending patent application Ser. No. 860,349, filed Sept. 23, 1969, now abandoned, which in turn is a continuationin-part of patent application Ser. No. 770,964, filed Oct. 28, 1968 which has been abandoned and replaced with copending continuation-in-part patent application S61. No. 75,009, filed Sept. 24, 1970.

BACKGROUND OF THE INVENTION This invention relates to the recovery of hydrocarbons and optionally water-soluble mineral from underground oil shale formations containing water-soluble mineral deposits. Further, the invention relates to hydrocarbon recovery by in-situ thermal fluid extraction of oil shale within a fractured and/or rubblized portion of a subterranean oil shale formation in and around a cavern and/or interconnected cavities formed by leaching or dissolving, e.g., solution mining of the watersoluble minerals therefrom and optionally recovering said minerals for suitable use in industry. More particularly, it relates to oil'recovery by in-situ retorting and- /or kerogen conversion within a caved-in or rubblized portion of a subterranean oil shale formation in and around a cavern formed by leaching or solution mining of water-soluble minerals therefrom, such as nahcolite, trona, halite, mixtures thereof, etc.

DESCRIPTION OF THE PRIOR ART Large deposits of oil in the form of oil shale are found in various sections of the United States, and particularly, in Colorado and surrounding states and in Canada. Various methods of recovery of oil from these shale deposits have been proposed and the principal difficultywith these methods is the highcost which renders the recovered oil too expensive to compete with petroleum crudes recovered by more conventional methods. The in-situ retorting or conversion of oil shale to recover the oil contained therein is made difficult because of the non-permeable nature of the oil shale and the difficulty of applying heat thereto without extensive mining or drilling operations. The mining and removal of the oil shale for retorting of the shale in furnaces outside the formation is commercially uneconomical in many cases.

In some subsurface oil shale formations such as in certain areas of the Green River formation in the Colorado and surrounding. areas of the United States, such formations also contain water-soluble minerals such as halite, nahcolite, trona, etc. in the form of beds, lenses, nodules, iodes, veins, or the like. Such minerals can be leached, dissolved and removed by suitable means such as solution mining, e,g., by contacting such minerals with a solvent such as, an aqueous solution which may be neutral, alkaline or acidic and recovering if desired, the mineral-containing solution and at the surface extracting the minerals from the solution andreusing the shale in the cavern can be induced from which hydro carbons can be recovered by suitable in-situ thermal means.

The art discloses various means of removing hydrocarbons, e.g., oil from oil shale as described, for example, in U. S. Pat. Nos. 3,400,762; 3,437,378; or 3,478,825 and also various means of increasing permeability of oil shale formations are described in US. Pat. Nos. 3,273,649; 3,481,398; and 3,502,372; or copending applications Ser. No. 829,350, filed July 7, 1969, now abandoned; or Ser. No. 770,964, filed Oct. 8, 1968, now abandoned and Ser. No. 75,009, filed Sept. 24, 1970 or Ser. No. 835,323, filed June 23, 1969 or Ser. No. 138,021, filed Apr. 28, 1971. Although these references are directed to an advancement of the art, the basic technique for recovering oil from oil shale still requires rubblization techniques such as, by means of explosive devices, e.g., nuclear energy which are expensive, difficult to control, and presents a radioactive contamination problem, all of which are very undesirable.

SUMMARY OF THE INVENTION It is an object of this invention to provide an improved method of recovering hydrocarbons from a water-soluble mineral-containing oil shale formation by leaching, dissolving or solution mining the minerals so as to form a cavern and/or interconnected cavities within the oil shale formation and simultaneously or sequentially flowing a hot fluid into the top of the cavern so as to place the roof under stresses of compression and/or tension and thereby effecting spalling and rubblization of the oil shale and thermally extracting and recovering hydrocarbons from the rubbled oil shale.

It is an object of this invention to provide a method of recovering hydrocarbons from oil shaleb y pretreab ing the formation by solution mining water-soluble minerals to form a cavern within a subterranean oil shale formation having a predominantly areally extensive void space therein sufficient to place the roof thereof under stresses of tension and/or compression.

It is a further object of this invention to provide an in-situ retorting process for an oil shale formation pretreated by solution' mining water-soluble minerals so as to create or establish a cavern therein and thereafter flowing a hot fluid near the top of the cavern so as to place the roof of the cavern in tension to effect cavingin oil shale in the vicinity of the cavern so as to establish improved fluid flow channels through which the in-situ retorting by a pyrolyzing fluid is conducted and thereby effect improved oil recovery.

These and other objects are attained and hydrocarbons are effectively recovered from oil shale formations containing deposits of soluble minerals by leaching, dissolving or solution-mining the minerals so as to establish interconnected voids or cavern having a predominantly areally extensive void space therein sufficient to place the roof thereof under stresses of compression and/or tension to effect spalling and caving-in by rubbling oil shale within the portions of the oil shale formation surrounding the cavern so as to displace rubbled oil shale into the cavern and thereafter flowing a kerogen-pyrolyzing fluid'through the cavern to effect recovery of shale oil from the formation. This may be effectively and preferably accomplished by (a) determining the location of zones rich in substantially watersoluble minerals within an oil shale formation; (b) extending at least one well borehole into the formation in communication with a zone rich in the soluble minerals; (c) establishing fluid communication between at least one well borehole and the soluble minerals at at least two spaced locations within the zone rich in the soluble minerals; (d) circulating a mineral dissolving or extracting fluid from one of the borehole locations through the minerals therebetween, and out the other of the borehole locations so as to dissolve the minerals and form interconnected voids or a cavern within the oil shale formation having a predominantly extensive void space therein sufficient to place the roof thereof under stress; (e) caving-in and rubbling oil shale within portions of the oil shale formation surrounding the cavern by circulating hot fluid through the cavern along flow paths controlled to keep the hot fluid in contact with the roof of the cavern and place it under stress; and (f) flowing a kerogen-pyrolyzing fluid from one of the borehole locations through the cavern within the oil shale formation and out the other of the borehole locations and recovering shale oil from the outflowing fluid.

Water-soluble minerals present in the oil shale is meant to include water-soluble silicates, halides, carbonates, and/or bicarbonates salts, such as alkali metal (Na, K) chloride, carbonate, bicarbonate and silicate, e.g., halite, trona, nahcolite and the like.

The process of the present invention is so designed so as to create a cavern or interconnecting cavities in the water-soluble mineral beds(s) or zone(s) by dissolving, leaching or solution mining techniques through at least one borehole penetrating said formation. Leaching can be effected by cold or hot fluids or solutions either at atmospheric or elevated pressures. When hot solutions are used such as hot water or acidified hot water and/or steam, more rapid dissolution is effected of certain water-soluble minerals such as nahcolite, trona, halite to produce void spaces in the oil shale formation thereby providing and enhancing well communication, space for thermal expansion of the shale, and greater surface for contact with subsequent pyrolyzing fluid. Water can be cold or hot or steam or any other aqueous fluids can be used such as steam and/or hot water containing acids, e.g., HCl, or HCl HF, surfactants, sequestering agents, etc. If the initial cavities are not in communication, fracturing may be necessary using such means as hydrofracturing, explosive means, nuclear means, etc., may be desirable. The leaching solutions can contain chemical agents to enhance dissolution of the minerals. Under certain leaching conditions decomposition of certain water-soluble minerals, e.g., bicarbonates, into solublizing materials may take place of such minerals as dawsonite and silicates which might be present in the formation, thereby increasing the porosity of the formation. For example, when nahcolite is dissolved with water, the pH of the dissolution fluid is increased and thereby aids in the dissolution of silicates, etc.

Leaching or solution mining of the water-soluble minerals such as halite or nahcolite can be accomplished by a suitable solution mining technique such as described in U.S. Pat. Nos. 2,618,475; 3,387,888; 3,393,013; 3,402,966; 3,236,564; 3,510,167 or Canadian Pat. Nos. 832,828 or 832,276 or as described in copending application Ser. No. 2,765 flied Jan. 14, 1970, now U.S. Pat. No. 3,596,992 Spalling and rubbling accomplished by hot fluid circulation into the top of the cavern causing the oil shale walls to spall, fracture, and rubble. ln-situ thermal recovery of oil is cffected by a pyrolyzing fluid such as steam or mixtures of hot water and steam or solvent extraction means.

The circulation of a pyrolyzing fluid into the top of the cavern and/or through the cavern not only effects oil recovery but also effects thermal rubbling and/or fracturization. Also, if the pyrolyzing fluid such as steam is used to extract and recover oil, more minerals may be dissolved perpetuating the process.

The term kerogen-pyrolyzing fluid is used to refer to a liquid or gas which, by means of thermal, chemical and/or solvent action, interacts with the kerogen components of the oil shale to produce and entrain hydrocarbons, e.g., oil. Such a fluid may comprise steam, hot hydrocarbons, hot gases, and/or mixtures of such fluids with chemicals such as acids and/or organic solvents. The kerogen-pyrolyzing fluid may be heated by surface or borehole-located heating devices and/or by means of in-situ combustion within the oil shale formation. The kerogen pyrolyzing fluid may advantageously comprise or contain a solvent for the soluble mineral, such as a steam condensate or a hot aqueous solution of organic and/or inorganic acid, having a temperature, such as at least several hundred degrees farenheit, such as from about 450 F to above about l,500 F and preferably from about 550 F to l,000 F. Where the kerogenpyrolyzing fluid forms aqueous components, its circulation through the treated oil shale formation may enlarge the cavern, by solution-mining the soluble mineral, while shale oil is being produced. The injection of oxygen-containing gas to initiate and sustain an under-' ground combustion, to form a kerogen-pyrolyzing fluid in situ, comprises a procedure for use in the present invention.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a vertical sectional view, partly diagrammatic, of a preferred embodiment of the invention;

FIG. 2 is a vertical sectional view of the treatment of the cavern formed by leaching or solution mining of water-soluble minerals as shown in FIg. l;

FIG. 3 is a sectional view of an embodiment of the invention, the formation being penetrated by a single well.

DESCRIPTION OF THE PREFERRED EMBODIMENT It has been found that it is economically feasible to locate and utilize a horizontally extensive zone of water-soluble mineral within a relatively impermeable subterranean earth formation, such as an oil shale formation.

As illustrated in FIG. 1, oil shale formation 12 contains a generally horizontally extensive bed 13 of a continuous deposit of a water-soluble mineral and nodules of minerals 12a, such as halite, nahcolite or trona. Such mineral containing areas are frequently encountered near the bottom of a layer of oil shale and may be utilized to form cavities or caverns, in accordance with the teachings of this invention, into which oil shale solids can be moved by caving-in of the'oil shale within formation 12.

In carrying out the method of this invention, the location of zones rich in substantially water-soluble minerals may be determined by geologic investigations based on an analysis of geophysical and geochemical data.

For example, the location and extent of such zones may be determined by obtaining and correlating seismic,

gamma ray and/or density log data, core analysis data, etc. The borehole of at least one well is extended into a zone containing a soluble mineral and fluid communication between a surface location and at least two spaced points within a substantially continuous and generally horizontally extensive zone rich in soluble mineral is established through the borehole or boreholes or the one or more wells.

Fluid communication is thus established between at least two spaced portions of well boreholes and the water-soluble minerals (as for example, through perforations 22 through 23 of well boreholes and 11, respectively, and the bed 13 of water-soluble minerals in FIG. 1). The well boreholes 10 and 11 of FIG. I extend into the soluble mineral bed 13 located within oil shale formation 12. The fluid communication between well boreholes 10 and 11 and the bed 13 in water-soluble minerals therebetween may be established by solutionmining a flow channel through the soluble mineral bed 13 and/or by means of conventional hydraulic, electric, and/or explosive fracturing techniques, all well known in the art. Where, for example, the subterranean stresses in and around soluble mineral bed 13 are conducive to the formation of horizontal fractures, the fluid communication between well boreholes 10 and 11 and the soluble minerals bed 13 may be established by a conventional hydraulic fracturing technique.

Where the subterranean stresses are conducive to the formation of vertical fractures, special steps are needed to form fractures that will interconnect pairs of spaced wells with each other and the soluble mineral. Such interconnecting fractures may be provided by initially forming parallel vertical fractures in each of a pair of wells. When at least one of the fractures is heated until it becomes closed, by a thermally-induced swelling of the fracture walls, a differently oriented vertical fracture is formed at a higher temperature and pressure. Such fractures are then extended until they intersect, and, the walls of the intersecting fractures are leached, for example, in the zone of their intersection with the soluble minerals. Such interconnected vertical fractures provide flow channels for circulating a soluble mineral-extracting solvent to form the caverns into which shale solids can be displaced prior to or during the pyrolysis of the kerogen. Such flow channels may be formed between a single pair of wells, to form a test pattern which can subsequently be expanded, or may be formed between all members of an extensive pattern of wells.

Alternatively, in similar tectonic environments, such interconnecting fractures may be formed by a combination of steps comprising (a) injecting a fluid through each of a pair of wells intothe soluble mineral-rich portion of the oil shale formation at a pressure sufficient to form and extend generally vertical fractures within the formation so that at least one pair of generally vertical fractures are extended within the formation for significant distances from the points of injection; (b) fllling the first-formed fracture in one of the wells with a fluid capable of solidifying in situ to a compressionresistant solid and allowing that fluid to solidify at substantially the pressure at which the first-formed fracture was extended; and (c) injecting a fluid through the well containing the so-plugged fracture at a pressure sufficient to form and extend a second-formed generally horizontal fracture through the oil shale formation until fluid injected through this well is transmitted to the other one of the pair of wells. Thus, by means of this procedure a low-cost fluid that is capable of solidifying in-situ, e.g., a slurry of cement or a solution of a resin, may be used to interconnect at least a pair of wells thereby aiding in solution-mining and shale oil production.

After fluid communication has been established between well boreholes 10 and 11, aqueous leaching or solution-mining fluid and/or liquid is injected down tubing string 24 and annulus outlet 30 in well borehole 10, out perforations 20 and 22, through bed 13 and up tubing string 25 and outlet 31 in well borehole 11 via perforations 21 and 23. The aqueous fluid and/or liquid may comprise water and/or steam or aqueous solutions of acid or acid-forming materials and is circulated at pressures either above or below the over-burden pressure. The circulating aqueous liquid dissolves the water-soluble minerals and mineral by-products thereof are recovered from the fluid flowing out of well borehole 1 1, for example, by conventional evaporation and- /or precipitation procedures. Thus, the solution-mining is preferably conducted by injecting hot water or steam or mixture of steam and aqueous liquid into the earth formation while producing fluids from the earth formation at a rate such that most or all of the produced fluid is liquid.

During the solution-mining operation described herein above, the depths at which the circulating aqueous liquid is injected into and produced from the earth formation are relatively immaterial as long as fluid is flowed through the water-soluble bed 13.

The solution-mining is continued. until the removal of water-soluble material has created a cavern 32 (FIG. 2) having a significantly large volume and areal extent and containing exposed oil shale along its roof 33. Such a cavern 32 should extend over a significant distance, such as at least about 50 feet or more along at least one horizontal direction while extending over an area of at least several hundred square feet and having a volume of at least several hundred cubic feet. Thus, as illustrated in FIG. 2, cavern 32 is formed within bed 13 extending to substantially the top of underlying formation 19 and the bottom of overlying oil shale formation 12.

Oil shale 34 (FIG. 2) from roof 33 is caved into the cavern 32 and the cavern roof 33 is migrated up within the oil shale formation 12 by circulating a kerogen pyrolyzing fluid through the cavern 32. The circulation of such a fluid is controlled by adjusting the points of injection and production and the temperature and pressure of the circulating fluid so that the roof 33 of the cavern 32 is contacted by kerogen-pyrolyzing fluid at a temperature at which significant amounts of the kerogen in the oil shale 34 are converted to fluid and removed from the cavern 32. In the initial stage this can be accomplished by injection down annulus outlet 30 and tubing string 24 of well borehole l0,.through cavem 32 into contact with the roof 33 thereof, and into well borehole 11 and out the annulus outlet 31 thereof.

Such a fluidization of kerogen tends to fracture the oil shale 34 since it both increases the volume of at least one component within the normally impermeable oil shale 34 and removes a significant proportion of solid material from the oil shale 34. In oil shale, the kerogen tends to be concentrated along layers such as bedding planes, and the effects of such kerogen fluidizing and removing actions tend to be localized, e.g., along the layers.

In the present process, the rate of the fracturing, flaking, or spalling of oil shale material into the cavern 32 is increased by the fact that the oil shale 34 being treated is under stresses of compression and/or tension along the roof 33 of cavern 32. The caving of oil shale 34 into the cavern 32 is further enhanced by controlling the circulating kerogen-pyrolyzing fluid so that the roof-contacting portion has a temperature above 450 F and preferably between 550 and 850 F at which kerogen is pyrolyzed at a relatively rapid rate. Since heat is transferred within the oil shale 34, the heat causes portions of kerogen which are surrounded by impermeable oil shale material to be converted to a liquid or vapor at pressures that fracture the surrounding oil shale material. Since this material is under stress (tension or compression) the fractured portions separate and cave into the cavern 32. Such a caving is further enhanced when the roof is contacted with an aqueous fluid, such as steam or hot aqueous liquid, that tends to dissolve and remove water-soluble material 35, such as entrapped or dispersed halite, nahcolite, or the like, from within the oil shale formation 12. The dissolving and removing of solid material is beneficial in both enhancing the caving and creating additional void space into which additional amounts of oil shale can be caved.

As the roof of the cavern 32 is migrated upward within the oil shale formation 12, the depth at which the circulating kerogen-pyrolyzing fluid is injected is preferably raised to keep it near the roof 33 of the cavern 32. This causes the roof 33 to be contacted by the hottest and most dilute incoming portions of the fluid. By similarly raising the depth of the points at which fluids are produced from the cavern 32, the circulating fluid is caused to flow along the roof 33 of the cavern 32 as a stream that flows over the top of a body of substantially static fluid within the cavern 32. The tendency to form such a flow path is enhanced when the circulating fluid is heated at a surface location or within well borehole 10 prior to its injection into the cavern 32. This causes the incoming fluid to be hottest and less dense than the fluid in the lower portion of the cavern 32. Such an effect is further enhanced by circulating a vapor-containing hot fluid, such as steam, hot gas, or a mixture of steam or hot gas and a liquid. The cavern 32 extends substantially to the top of oil shale formation 12.

The kerogen-pyrolyzing fluid continues to be circulated from well borehole 10 through the cavern 32 of oil shale formation 12 and out of well borehole 11 in the manner discussed hereabove with respect'to FIG. I. Hydrocarbons are continuously recovered from the heated fluid circulating out of well borehole II by means well known in the art.

Thus, as illustrated in FIG. 1, conventional equipment and techniques, such as heating means, pumping means, separators and heat exchangers may be used for pressurizing, heating, injecting, producing, and separating components of the heated fluid circulating through the cavern 32 within oil shale formation 12. The production of the fluid may be aided by downhole pumping means, not shown, or restricted to the extent necessary to maintain the selected pressure within the oil shale formation 12.

The fluid circulated through cavern 32 to recover hydrocarbons from oil shale formation 12 may comprise any heated fluid and/or liquid, such as hot gases, hot

water, and mixtures thereof. Oil shale reactive properties may also be imparted to the circulating fluid as discussed hereinabove.

Numerous methods are known and available to those skilled in the art for arranging conduits in the portions of the well boreholes that are adjacent to such watersoluble zones and the overlying oil shale so that (a) fluids can be injected and produced at increasingly shallower depths and (b) the well conduits are unaffected by the collapsing of the earth formations.

As shown in FIG. 1 of the drawing, the well boreholes may be suitably arranged by casing such portions of the well boreholes and cementing the casings at locations above and below the intervals to be treated. Such casings are preferably equipped with means, such as slidable and/or collapsible elements, for avoiding buckling or overstressing due to thermally induced expansion or contraction. Such casings may be perforated along the interval to be exploided at substantially any convenient time with substantially any density of openings.

In using the illustrated well equipment, the treatment of the water-soluble zone may be started prior to casing or fracturing the wells, for example, by underreaming the well boreholes and/or jetting fluid into the watersoluble zone to radially extend a notch around the well. After the wells have been cased and perforated, they are preferably fractured in order to establish an interwell communication through fracture paths. Numerous procedures are known and available for accomplishing such fracturing and various suitable procedures are described in my copending patent application and the applications referred to therein.

The solution-mining operation is preferably conducted by circulating hot fluid or aqueous liquid through a well-interconnecting flow path. The flow of a hot fluid, e.g., steam or aqueous fluid, tends to be confined within the water-soluble zone even if intercommunicating fractures have extended into the overlying oil shale. Within the oil shale, the fractures tend to be closed by the thermal swelling of the fracture walls. Within the water-soluble zone, such a closure is prevented by the dissolving of the wall material.

After forming cavern 32 along the bottom of the oil shale, the roof 33 of the cavern 32 is migrated up within the oil shale by circulating a kerogen-pyrolyzing fluid along the roof 33 of the cavern 32. The kerogenpyrolyzing fluid may be the same fluid used in the solution-mining operating and roof-migrating operations may, to some extent, be concurrent.

The migration of the cavern roof 33 up within the oil shale formation 12 forms a fracture-permeated zone that may contain a significant amount of recoverable hydrocarbons. Hydrocarbons may be recovered from this zone by substantially any oil shale pyrolyzing procedure such as an in-situ combustion, hot fluid heating, or the like type of subterranean retorting procedure. Beyond the areal extent of the cavern 32, the oil shale 34 tends to remain impermeable and thus tends to com fine any fluids which are being circulated through the fracture-permeated zone.

A single well system shown in FIG. 3 may be utilized by a dual zone completion arrangement such that fluids can be injected at one point of the well and produced from another point of the same well. This fluid communication can be established in one borehole between at least two spaced portions of the well borehole and the water-soluble minerals (as for example, in FIG. 3 communication is through the tubing string the ends of which are open to the water-soluble minerals and some distance apart.) Thus a single well may be utilized by a dual zone completion arrangement as shown in FIG. 3 such that fluids can be injected at one point of the well and produced from another point of the same well. In FIG. 3, the welbore is 40, the casing is 41, the sealant is 47, within the casing arethe injection tubing string 43 and production tubing string 44, the borehole 40 penetrates oil shale formation 9 with mineral zone(s) 10 or multizones 10b and 100 and mineral nodules 48.

Fracturing pressures are generated within the oil shale formation 9 while lower pressures are maintained within the cavern 45 which is formed within oil shale formation 9 by the removal of the water-soluble minerals. These pressures are preferably generated by merely circulating hot fluid through cavern 23a. As the roof of v the cavern 45 is heated kerogen is pyrolyzed within the cavern walls and the pressures of the pyrolysis products increase until portions of the walls are spalled into the cavern 45 creating a rubblized zone 24a and surrounding fracture area 46.

In summary, then, well boreholes are opened into at least two horizontally spaced points within such a water-soluble zone and are equipped to convey fluids between that zone and at least one surface location. Fluid circulation is established between the spaced points within the soluble zones An aqueous fluid is circulated through that zone in order to solution-mine a cavern having a roof along which oil shale is exposed. The cavcm is extended to form a void space having horizontal dimensions sufficient to create a tensile stress, due to the tendency of the roof to sag, and to have a volume sufficient to accommodate a significant amount of solid material. Oil shale is caved into the cavern by circulating a kerogen-pyrolyzing fluid through the cavern at a temperature at which kerogen in the oil shale is converted to fluid material that is extracted and removed by the circulating fluid. The depths and-rates at which the circulating fluid is injected into and produced from the cavern are adjusted to maintain a flow along the roof as the roof migrates up within the oil shale formation.

EXAMPLES (A) A single injection-production well was designed properly insulated to penetrate a nahcolite rich oil shale formation and through the injection tubing hot water was injected into the nahcolite bearing formation for about a week to leach out the nahcolite as an aqueous solution which was recovered above ground via the production tubing. The size of the cavern thus produced was determined by conventional means such as measuring the concentration of the salt in solution and thereafter steam at a temperature above 500 F was injected along the roof of the cavern to effect roof stress resulting in rubbling and spalling of the oil shale into the cavern and cavern enlargement. Steam injection along the cavern roof was continued as well as through the rubbled oil shale bed at a temperature between 550 F and 850 F effecting kerogen decomposition to hydrocarbons which were recovered via the production tubing where the hydrocarbons were separated above ground from the condensed steam by conventional means.

(8) The procedure as described in (A) was repeated except that instead of using hot water to leach out the nahcolite steam was used. Thus, once the cavern was formed by steam leaching out the nahcolite, steam injection was continued under controlled conditions so that its flow path was directed along the roof of the cavern placing it under stresses of tension and compression producing results as noted in (A). Thereafter a portion or the entire steam injection flow was directed to flow through the rubbled oil shale to effect kerogen decom position and hydrocarbon recovery as noted in (A).

Other than steam flow control and direction the (B) process can be regarded as a single process operation in which steam is used as the leaching, rubbling and hydrocarbon recovery agent provided temperature, pressure and flow control of the steam as described in (B) is carefully controlled in accordance with the teaching of the present invention.

The method described hereinbelow of caving oil shale into a void space within a subterranean oil shale provides the advantages of l converting at least some of the kerogen to hydrocarbon products that can be re-. covered and sold while the process is being operated, (2) converting at least some of the kerogen to fluid that can be used to form a part or all of a kerogenpyrolyzing fluid that is circulated to operate such a process, and (3) caving the .oil shale above an areally extensive void space which was formed by a solutionmining process, (that produced a valuable by-product) and thus converting the relatively impermeable oil shale into rubble from which shale oil can be recovered.

We claim as our invention:

1. A method of recovering hydrocarbonsfrom a subterranean oil shale formation rich in water-soluble minerals comprising creating a cavern in the mineral zone of the formation by solution dissolving the watersoluble mineral, placing the roof of the cavern under stress, by circulating a hot fluid along the roof of the cavern so as to effect spalling and rubbling of oil shale cavern roof into the cavern while effecting pyrolysis with a hot pyrolysis fluid of the rubbled oil shale and recovering hydrocarbons therefrom.

2. The method of claim 1 wherein the water-soluble mineral dissolving solution is an aqueous fluid and the hot fluid circulating along the cavern roof effecting spalling and rubbling of the oil shale and the hot pyrolysis fluid is steam.

3. The method of claim 2 wherein the aqueous fluid is hot water.

4. The method of claim 3 wherein the water-soluble mineral is nahcolite.

5. The method of claim 2 wherein the aqueous fluid is steam.

6. The method of claim 5 wherein the water-soluble mineral is nahcolite.

7. The method of claim 1 wherein the water-soluble mineral are water-soluble carbonates.

8. The method of claim 7 wherein the carbonate is nahcolite.

9. In a method for producing oil from a subterranean oil shale formation containing an adjacent underlying zone of rich water-soluble minerals comprising the steps of:

a. determining the location of said zone that is rich in water-soluble minerals within said oil shale formation;

b. extending at least one well borehole into the watersoluble mineral containing zone and establishing communication between the well borehole and the I water-soluble mineral containing zone;

0. dissolving the water-soluble minerals by circulating therein via the borehole and into the mineralcontaining zone an aqueous dissolving liquid and creating a fluid-filled cavern having a predominantly areally extensive void space therein sufficient to place the roof of the cavern in tension;

d. circulating a hot fluid through the cavern along flow paths controlled to keep the hot fluid in contact with the roof of the cavern causing oil shale in contact and adjacent to the cavern roof to rubble and cave into the cavern;

e. flowing an in-situ oil recovering fluid into said rubblized oil shale cavern (d); and

f. recovering shale oil therefrom.

10. The method of claim 9 wherein communication step (b) is effected by fracturing.

11. The method of claim 9 including the step of recovering mineral by-products from the aqueous liquid circulating out of the other of said borehole locations.

12. The method of claim 9 including the steps of:

providing at least said one well borehole with casing extending along said zone and into said earth formations at least a short distance below said zone; and

providing said casing with slip joint means for varying the length thereof.

13. The method of claim 9 including the step of circulating hot aqueous fluid into contact with the roof of said cavern while flowing said kerogen-pyrolyzing fluid through said cavern.

14. The method of claim 9 wherein the step of circulating aqueous liquid through said zone to leave a cavern therein includes the step of circulating aqueous liquid through said zone until a cavern is formed therein extending at least 50 feet or more along at least one horizontal direction while extending over an area at least several hundred square feet and having a volume of at least several hundred cubic feet.

15. In a method for producing oil from a subterranean oil shale formation containing an adjacent underlying zone of rich water-soluble minerals comprising the steps of:

a. determining the location of the rich-water-soluble mineral zone within the oil shale formation,

b. extending at least a pair of well boreholes within the zone rich in water-soluble minerals;

c. establishing fluid communication between at least one pair of said wells by injecting an aqueous liquid via each of said boreholes to a point intermediately between said boreholes;

d. circulating aqueous liquid through said zone that is rich in water-soluble minerals to dissolve said minerals and forming a fluid-filled cavern within the oil shale formation having a predominantly areally extensive void space therein sufficient to place the roof thereof in tension;

e. caving oil shale within the portions of said oil shale formation communicating with said cavern into said cavern by circulating hot fluid through the cavern along flow paths controlled to keep the hot fluid in contact with the roof of the cavern;

f. flowing a kerogen-pyrolyzing fluid from one of said locations to another through the cavern within the oil shale formation to effect shale oil recovery from said oil shale adjoining and within said cavern.

16. The method of claim 15 wherein the aqueous liquid in (c) is water and the fluid in (e) is a kerogenpyrolyzing fluid.

17. The method of claim 16 wherein the water is hot water and the pyrolyzing fluid is steam.

18. The method of claim 17 wherein the watersoluble mineral is nahcolite. 

2. The method of claim 1 wherein the water-soluble mineral dissolving solution is an aqueous fluid and the hot fluid circulating along the cavern roof effecting spalling and rubbling of the oil shale and the hot pyrolysis fluid is steam.
 3. The method of claim 2 wherein the aqueous fluid is hot water.
 4. The method of claim 3 wherein the water-soluble mineral is nahcolite.
 5. The method of claim 2 wherein the aqueous fluid is steam.
 6. The method of claim 5 wherein the water-soluble mineral is nahcolite.
 7. The method of claim 1 wherein the water-soluble mineral are water-soluble carbonates.
 8. The method of claim 7 wherein the carbonate is nahcolite.
 9. In a method for producing oil from a subterranean oil shale formation containing an adjacent underlying zone of rich water-soluble minerals comprising the steps of: a. determining the location of said zone that is rich in water-soluble minerals within said oil shale formation; b. extending at least one well borehole into the water-soluble mineral containing zone and establishing communicaTion between the well borehole and the water-soluble mineral containing zone; c. dissolving the water-soluble minerals by circulating therein via the borehole and into the mineral-containing zone an aqueous dissolving liquid and creating a fluid-filled cavern having a predominantly areally extensive void space therein sufficient to place the roof of the cavern in tension; d. circulating a hot fluid through the cavern along flow paths controlled to keep the hot fluid in contact with the roof of the cavern causing oil shale in contact and adjacent to the cavern roof to rubble and cave into the cavern; e. flowing an in-situ oil recovering fluid into said rubblized oil shale cavern (d); and f. recovering shale oil therefrom.
 10. The method of claim 9 wherein communication step (b) is effected by fracturing.
 11. The method of claim 9 including the step of recovering mineral by-products from the aqueous liquid circulating out of the other of said borehole locations.
 12. The method of claim 9 including the steps of: providing at least said one well borehole with casing extending along said zone and into said earth formations at least a short distance below said zone; and providing said casing with slip joint means for varying the length thereof.
 13. The method of claim 9 including the step of circulating hot aqueous fluid into contact with the roof of said cavern while flowing said kerogen-pyrolyzing fluid through said cavern.
 14. The method of claim 9 wherein the step of circulating aqueous liquid through said zone to leave a cavern therein includes the step of circulating aqueous liquid through said zone until a cavern is formed therein extending at least 50 feet or more along at least one horizontal direction while extending over an area at least several hundred square feet and having a volume of at least several hundred cubic feet.
 15. In a method for producing oil from a subterranean oil shale formation containing an adjacent underlying zone of rich water-soluble minerals comprising the steps of: a. determining the location of the rich-water-soluble mineral zone within the oil shale formation, b. extending at least a pair of well boreholes within the zone rich in water-soluble minerals; c. establishing fluid communication between at least one pair of said wells by injecting an aqueous liquid via each of said boreholes to a point intermediately between said boreholes; d. circulating aqueous liquid through said zone that is rich in water-soluble minerals to dissolve said minerals and forming a fluid-filled cavern within the oil shale formation having a predominantly areally extensive void space therein sufficient to place the roof thereof in tension; e. caving oil shale within the portions of said oil shale formation communicating with said cavern into said cavern by circulating hot fluid through the cavern along flow paths controlled to keep the hot fluid in contact with the roof of the cavern; f. flowing a kerogen-pyrolyzing fluid from one of said locations to another through the cavern within the oil shale formation to effect shale oil recovery from said oil shale adjoining and within said cavern.
 16. The method of claim 15 wherein the aqueous liquid in (c) is water and the fluid in (e) is a kerogen-pyrolyzing fluid.
 17. The method of claim 16 wherein the water is hot water and the pyrolyzing fluid is steam.
 18. The method of claim 17 wherein the water-soluble mineral is nahcolite. 